1. Field of the Invention
The present invention relates to an image observation apparatus such as a head-mounted display, a glasses-type display, a camera finder, or the like.
2. Description of the Related Art
An image observation apparatus such as a head-mounted display or the like is equipped with an observation optical system for projecting an enlarged virtual image of an image displayed on an image display device to an observer""s eye, and conventionally, various observation optical systems are proposed for downsizing the entirety of the device.
For example, in Japanese Patent Laying-Open No. H07-333551 (corresponding U.S. Patent Application Publication No. 20010009478), an observation optical system is proposed, wherein a prism-shaped projecting optical element is used, which is comprised of first, second and third surfaces all having rotational asymmetric shapes, and has a symmetry shape for one symmetry plane, and an image displayed on an image display device is projected to an observer""s eye as an enlarged image.
Use of this prism-shaped optical element allows an extremely compact and simple construction to sufficiently correct image distortion, curvature of the image plane, and astigmatism.
However, in such an observation optical system, when the angle of light to be guided to the eye (angle of view) is set to be great, chromatic aberration occurs due to light refraction when the light exits from the exit surface at the eye side. However, if such a prism-shaped projecting optical element is composed of a single material, so-called chromatic alternation correction is difficult. Therefore, it has been conventionally difficult to correct chromatic aberrations.
Accordingly, a construction for an observation optical system is proposed in Japanese Patent Laying-Open No. H09-65246 (corresponding U.S. Pat. No. 5,768,025) wherein a diffraction surface having a dispersion property in reverse to that of the refractive surface is used for a part of the optical system so as to correct chromatic aberrations occurring at the refractive surface.
FIG. 24 and FIG. 25 show an observation optical system having the prism-shaped projecting optical element P disclosed in the abovementioned Japanese Unexamined Patent Publication No. H09-65246. In these figures, E shows an observer""s pupil, and I shows an image display device. In FIG. 24, 101 denotes a first surface of the projecting optical element P, 103 (104) denotes a second surface, 102 denotes a third surface, and 112 denotes a diffraction optical element. On the other hand, in FIG. 25, 121 denotes a first surface of the projecting optical element P, 123 (124) denotes a second surface, 122 denotes a third surface, and 125 denotes a diffraction optical element.
As shown in FIG. 24, the construction in which the diffraction optical element 112, which is completely separate from the projecting optical element P, is additionally provided at the eye (pupil E) side of the observation optical system, not only increases the number of components of the apparatus, but also requires control with high accuracy of the space and positions of the diffraction optical element 112 and projecting optical element P, and causes the shapes of members for holding the diffraction optical element 112 and projecting optical element P to become complicated and increase in size.
Furthermore, as shown in FIG. 25, if the diffraction optical element 125 is provided at the incidence surface 121 of the projecting optical element P, normally, Fno. of the observation optical system becomes greater (darker), and the focal depth increases, so that a discontinuous shape of the diffraction surface is exposed to an observer""s eye.
In order to solve these problems, in the invention, in an image observation apparatus, which is comprised of an image display element for displaying images, and a plurality of optical action surfaces including a first refractive surface on which image light from the image display element is made incident, a reflective surface for reflecting the image light made incident from this first refractive surface, and a second refractive surface for emitting the image light reflected by this reflective surface, and a projecting optical element for guiding the image light to an observer""s eye, a diffraction optical part having a diffraction effect is provided in an integral manner with the optical element on the reflective surface or in the vicinity of the reflective surface.
Thereby, chromatic aberrations in an image due to light refraction at the first and second refractive surfaces (particularly, chromatic aberrations at the peripheral portion) can be satisfactorily corrected without control with high accuracy in the space between the diffraction optical element and projecting optical element and without causing the shapes of members for holding the diffraction optical element and projecting optical element to become complicated and increase in size.
Furthermore, in the case where the second refractive surface is a surface for reflecting image light, which has been made incident inside the optical element from the first refractive surface, toward the reflective surface, and emitting the image light reflected by this reflective surface toward the outside of the optical element, that is, the second refractive surface has a plurality of functions including reflection and transmission (particularly, when the reflection region and transmission region overlap each other), it is difficult to provide a diffraction optical part on this second refractive surface in terms of design. Furthermore, if a diffraction optical part is provided on the first refractive surface as an incidence surface, a discontinuous shape of the diffraction surface is exposed to the observer""s eye as mentioned above. Therefore, the provision of a diffraction optical part on (or in the vicinity of) the reflective surface makes it possible to correct chromatic aberrations in images without problems such that the shape of the diffraction surface is exposed to the observer""s eye.
In addition, although the height of the step of the diffraction grating becomes equal to the wavelength xcex of the image light when a diffraction optical part is provided on the refractive surface through which image light transmits, the provision of the diffraction optical part on the reflective surface makes it possible to set the height of the step of the diffraction grating to xcex/2, and manufacturing of the grating (including mold manufacturing and cutting) becomes easier.
Furthermore, the reflective surface on which the diffraction optical part is provided desirably has the most strong optical power (1/f:reciprocal of the focal length) among the plurality of optical action surfaces of the optical element. Thereby, chromatic aberrations in an image can be effectively corrected.
Furthermore, when the base shape of the reflective surface is formed to be asymmetric without a symmetry axis, the diffraction optical part may be shaped so that the shape of the part is added to the base shape of this reflective surface. Thereby, aberrations other than chromatic aberrations occurring at the diffraction optical part are canceled by the base shape of the reflective surface, and only chromatic aberrations are effectively corrected by the diffraction optical part.
In addition, when the plurality of optical action surfaces of the optical element are constructed to be decentering systems, respectively, by forming the grating structure of the diffraction optical part to be asymmetric without a symmetry axis, decentering aberrations and asymmetric chromatic aberrations that occur when the optical actions surface are used in the decentering condition can be effectively corrected.
Moreover, the step-like grating structure comprising the diffraction optical part is directly formed on the reflective surface, and reflective layers are provided on the surface of this grating structure, whereby chromatic aberrations can be corrected by a simple construction.
Furthermore, the optical element may be constructed so that a grating structure comprising a diffraction optical part is formed at the surface of a first substance having the first refractive surface and the second refractive surface, the reflective surface is disposed in the vicinity of the grating structure surface, and a second substance, which is different from the first substance is filled between the grating structure surface and reflective surface. In this case, it is preferable that the first substance and the second substance are made to be different from each other in refractive index and Abbe""s number.
Thereby, the optical dispersion value of the medium differs between the front and rear of the grating structure surface, and the diffraction efficiency at the diffraction optical part can be totally increased while being made almost even in a used wavelength region.